[PrunedLiveness] Allow inhabited discovered blocks

The vector is just used for collecting blocks that have been discovered,
so it's fine to append such blocks to a list that's already not empty.

Author: nate-chandler

include/swift/SIL/PrunedLiveness.h

@@ -196,9 +196,7 @@ class PrunedLiveBlocks {
 public:
   PrunedLiveBlocks(SILFunction *function,
                    SmallVectorImpl<SILBasicBlock *> *discoveredBlocks = nullptr)
-      : liveBlocks(function, 2), discoveredBlocks(discoveredBlocks) {
-    assert(!discoveredBlocks || discoveredBlocks->empty());
-  }
+      : liveBlocks(function, 2), discoveredBlocks(discoveredBlocks) {}
 
   bool isInitialized() const { return initializedFlag; }